1. Field of the Invention
The invention relates to an echo canceller comprising a receive path and a send path, the two having each an input and an output, the output of the receive path being connected to the input of an adaptive filter comprising adapting means, which filter has an impulse response which is an estimate of the impulse response of an echo path between the output of the receive path and the input of the send path, the input of the send path being coupled to a first input of a subtractor circuit and the output of the adaptive filter being coupled to a second input of the subtractor circuit, and the output of the subtractor circuit being coupled to the output of the send path; this echo canceller also including detecting means which generates a control signal for blocking the adapting means if the detecting means detect a locally generated signal at the input of the send path.
2. Description of the Related Art
An echo canceller of this type is known from Dutch Patent Application 8701633 which corresponds to U.S. Pat. No. 4,903,247.
In full-duplex signal transmission the presence of an undesired echo path between the output of the receive path and the input of the send path, and the presence of such an echo path at the far-end may cause a closed signal loop to develop. Such a situation may occur, for example, in telephony with the aid of loudspeaking telephone sets.
Because amplifiers are included in the send path, loop gain for a specific frequency may be greater than 1. Consequently, a type of oscillation occurs also denoted acoustic feedback when speech transmission is concerned.
If the acoustic feedback is smaller than 1, oscillation will not occur, it is true, but after a certain delay an echo of the signal applied to the input of the send path will appear at the output of the receive path via the far-end echo path. In telephony this means that a speaker hears his own voice delayed by a specific period of time. This phenomenon is experienced as extremely annoying especially in case of long delays.
In order to suppress these echoes, controllable attenuators included in the send and receive paths are utilized in known fashion. In the case where only a near-end signal generated by a near-end speaker is present, the attenuator in the send path has a high transfer factor A.sub.max and the attenuator in the receive path a low transfer factor A.sub.min. In the case where only a signal coming from the far end is available, the attenuator in the receive path has a high transfer factor A.sub.max and the attenuator in the send path a low transfer factor A.sub.min. In the case where there is a near-end generated signal as well as a far-end generated signal available, the two attenuators are set to an equal transfer value which equals .sqroot.A.sub.max *A.sub.min. In all cases the loop gain is now reduced to such an extent that the problems caused by echo signals are strongly reduced. A problem with this type of echo cancelling is that the beginning of a speech signal is either not transferred or is transferred in a strongly attenuated manner, because after the detection of the speech signal a specific amount of time is required before the attenuators can be adjusted to a correct transfer factor. In addition, when doubletalk occurs, the transfer factor will be lower than necessary because the two loudspeaking telephone sets introduce an attenuation both in the send and receive paths. Consequently, when doubletalk occurs, the signal received from the far end will be so attenuated that the two speakers almost cannot hear each other any more.
A better method of transferring the beginning of a speech signal or speech signals in case of doubletalk, without affecting the echo cancelling, is to use an echo canceller either alone or in combination with adjustable attenuators inserted in the send and/or receive path. This echo canceller comprises an adaptive filter having an impulse response which is an estimate of the impulse response of the echo path between the output of the receive path and the input of the send path. Feeding the output signal of the receive path to the input of the adaptive filter causes an estimate of the echo signal to be available at the output of the adaptive filter.
By subtracting the output signal of the adaptive filter from the input signal of the send path, a difference signal is obtained which comprises only the near-end generated input signal of the send path in addition to the remainder of the echo signal.
The desired impulse response of the adaptive filter is calculated by the adapting means on the basis of the difference signal found. This is only feasible if the difference signal comprises only the remainder of the echo signal. If a near-end generated signal is present at the input of the send path, this near-end generated signal will also be present in the difference signal. Because the adapting means in the adaptive filter cannot make any distinction between a remainder of the echo signal and a near-end generated signal, a proper adaptation of the impulse response of the adaptive filter will be disturbed. In order to avoid this, the adapting means will be blocked if the detection means detect a near-end generated signal.
In the echo canceller known from afore-mentioned Dutch Patent Application, this detection is performed by detecting means which calculates the ratio of the signal power of the difference signal to the signal power of the input signal of the send path and comparing this ratio with a given threshold. If this ratio exceeds the threshold, a near-end generated signal is assumed to be present. This is because once the adaptive filter has been adjusted the ratio of the signal power of the difference signal (which signal is then equal to the residual signal) to the signal power of the input signal of the send path will be relatively small (order of magnitude: 0.01-0.001) when a locally generated signal is absent.
A general problem for echo cancellers is that the adaptive filter is unable to adapt its own impulse response sufficiently rapidly when there is a fast change of the impulse response of the echo path. Consequently, the ratio of the signal power of the difference signal to the signal power of the input signal of the send path strongly increases and the detection means could erroneously detect the presence of a near-end generated signal even when it is not actually present.